1. Field of the Invention
This invention relates to a motor-driven clamping method for driving a bolt or the like with a constant torque.
2. Description of the Prior Art
For example, in the assembling of an automobile or an internal combustion engine, an operation of driving a bolt into a workpiece and clamping it for rigidity is carried out at an extraordinary frequency and is very laborious; hence, it is achieved in many cases by an automatic clamping tool utilizing compressed air or electric power. A motor-driven tool, though usually heavy as compared with a pneumatic tool, is easy to handle and less noisy, and hence is widely employed. With a prior art motor-driven tool, the head of a bolt is inserted in a socket of a rotary shaft affixed to the tip of the tool and the bolt is rapidly driven into a workpiece by the rotary shaft (driven via a reduction gear, for example, at 500 rpm) with the rotation (for instance, 5000 rpm) of a motor. When the driving operation is almost complete and a load on the motor starts to increase suddenly, a mechanical clutch between the motor and the rotary shaft is disengaged to switch the rotary shaft to a low speed (for example, 5 rpm) and, when it is detected by a torque detector built in the tool that the torque applied to the rotary shaft has reached a certain value, the motor is stopped; in this manner, the bolt is clamped to the workpiece with a constant torque. As a consequence, the clamping torque can be made uniform but the weights of the built-in clutch, the device for mechanically detecting the torque to disengage the clutch and so forth are added to the weight of the motor, so that the total becomes heavy and inconvenient to handle.